Signal transmission circuit, electronic device, cable, and connector

ABSTRACT

A signal transmission circuit has first to fourth inductors, and first and second varistors. The first and second inductors are magnetically coupled to each other. The first varistor is located posterior to the first inductor and is electrically connected in parallel to the first inductor. The third inductor is located between the first inductor and the first varistor and is electrically connected in series to the first inductor. The second varistor is located posterior to the second inductor and is electrically connected in parallel to the second inductor. The fourth inductor is located between the second inductor and the second varistor and is electrically connected in series to the second inductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a signal transmission circuit and to anelectronic device, a cable, and a connector comprising the signaltransmission circuit.

2. Related Background Art

The differential transmission system is one of systems for transmittingdigital signals between electronic devices. The differentialtransmission system is a system of feeding mutually opposite digitalsignals on a pair of lines, and the differential transmission cancelsout radiated noise from signal lines, and external noise. Thecancellation of external noise results in decrease in noise, whichpermits transmission of signals in small amplitude. Furthermore, thesignals in small amplitude offer an advantage of reduction in rise andfall times of signals, which implements increase in speed of signaltransmission.

The interface standards using this differential transmission systeminclude USB (Universal Serial Bus), IEEE1394, LVDS (Low VoltageDifferential Signaling), DVI (Digital Video Interface), HDMI(High-Definition Multimedia Interface), and so on. Among these, HDMI isan interface enabling transmission of more digital signals, andhigh-speed interface enabling transmission of uncompressed digitalsignals between a source device (e.g., a DVD player, a set-top box, orthe like) and a sink device (e.g., a digital television set, aprojector, or the like). The HDMI permits high-speed transmission ofvideo signals and sound signals through a single cable.

Incidentally, the increase of transmission speed leads to generation ofnoise even with microscopic deviation of differential signals betweensignal lines. A solution to this problem is a proposal on a transmissioncircuit that reduces the noise by insertion of a common mode choke coilin an interface such as a cable (e.g., reference is made to JapanesePatent Application Laid-Open No. 2001-85118).

SUMMARY OF THE INVENTION

In the case of the high-speed interfaces such as HDMI, the structure ofIC itself becomes more susceptible to ESD (Electrostatic Discharge), inorder to realize the increase of speed. For this reason, there areincreasing demands for countermeasures against ESD in the high-speedtransmission type ICs, and capacitive elements such as varistors andZener diodes are used as ESD-prevention components.

However, insertion of capacitive elements as ESD-prevention componentsin the transmission lines was found to cause a new problem that signalsthrough the transmission lines, particularly, high-frequency (200 MHz orhigher) or high-speed pulse signals were reflected and attenuated. Thisresults from the fact that with insertion of the capacitive elements inthe transmission lines, the capacitance of the capacitive elementslowers the characteristic impedance at the position of insertion of thecapacitive elements in the transmission lines so as to cause animpedance mismatch at the position. The existence of theimpedance-mismatched portion in the transmission lines leads toreflection of high-frequency components of signals at the characteristicimpedance-mismatched portion and, in turn, to occurrence of return loss.This results in heavily attenuating the signals. In addition, thereflection can induce unwanted radiation in the transmission lines, soas to cause noise.

For the HDMI, specified values of the characteristic impedance of thetransmission lines (TDR standard) are defined as 100 Ω±15%(High-Definition Multimedia Interface Specification Version 1.1).

An object of the present invention is to provide a signal transmissioncircuit, an electronic device, a cable, and a connector capable ofsuppressing the reduction of the characteristic impedance even with useof capacitive elements as ESD-prevention components.

A signal transmission circuit according to the present inventioncomprises first and second inductors magnetically coupled to each other;a first capacitive element located posterior to the first inductor andelectrically connected in parallel to the first inductor; a secondcapacitive element located posterior to the second inductor andelectrically connected in parallel to the second inductor; a thirdinductor located between the first inductor and the first capacitiveelement and electrically connected in series to the first inductor; anda fourth inductor located between the second inductor and the secondcapacitive element and electrically connected in series to the secondinductor.

In the signal transmission circuit according to the present invention,the first to fourth inductors suppress the reduction of thecharacteristic impedance due to the first and second capacitiveelements.

Preferably, the signal transmission circuit further comprises a fifthinductor located posterior to the first capacitive element andelectrically connected in series to the third inductor; and a sixthinductor located posterior to the second capacitive element andelectrically connected in series to the fourth inductor. Thisconfiguration is able to further suppress the reduction of thecharacteristic impedance due to the first and second capacitiveelements.

An electronic device, a cable, and a connector according to the presentinvention comprise the foregoing signal transmission circuit.

Each of the electronic device, cable, and connector according to thepresent invention is able to suppress the reduction of thecharacteristic impedance due to the first and second capacitiveelements, as described above.

The present invention successfully provides the signal transmissioncircuit, electronic device, cable, and connector capable of suppressingthe reduction of the characteristic impedance even with use of thecapacitive elements as ESD-prevention components.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a signal transmission circuitaccording to the first embodiment.

FIG. 2 is a circuit diagram showing the signal transmission circuitaccording to the first embodiment.

FIG. 3 is a schematic diagram to illustrate an operation of a commonmode filter.

FIG. 4 is a schematic diagram showing a modification example of thesignal transmission circuit according to the first embodiment.

FIG. 5 is a schematic diagram showing another modification example ofthe signal transmission circuit according to the first embodiment.

FIG. 6 is a schematic diagram showing still another modification exampleof the signal transmission circuit according to the first embodiment.

FIG. 7 is a circuit diagram showing a signal transmission circuitaccording to the second embodiment.

FIG. 8 is an illustration for explaining a measurement environment bythe TDR method.

FIG. 9 is an illustration for explaining a measurement method by the TDRmethod.

FIG. 10 is a graph showing the measurement results by the TDR method.

FIG. 11 is a graph showing the measurement results by the TDR method.

FIG. 12 is a graph showing the measurement results by the TDR method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings. Identicalelements or elements with identical functionality will be denoted by thesame reference symbols in the description, without redundantdescription.

As shown in FIG. 1, a digital television set 1 and a DVD player 2 areconnected through HDMI cable 3. The HDMI cable 3 is a cable using thedifferential transmission system and is equipped with joining terminalparts 5, 6 (connectors). The joining terminal part 5 of HDMI cable 3 isconnected to an output part of DVD player 2. The joining terminal part 6of HDMI cable 3 is connected to an input part of the digital televisionset 1. Digital signals fed out of the DVD player 2 are transmitted athigh speed through the HDMI cable 3 to the digital television set 1.

The digital television set 1 is equipped with a signal transmissioncircuit SC1 in the input part thereof. The signal transmission circuitSC1, as shown in FIG. 2, is provided with a common mode filter 10 havingfirst and second inductors 11, 12 magnetically coupled to each other,third and fourth inductors 21, 22, and first and second varistors 31,32. The common mode filter 10 has input/output terminals 13, 14connected to the first inductor 11, and input/output terminals 15, 16connected to the second inductor 12. When the joining terminal part 6 ofthe HDMI cable 3 is connected to the input part of the digitaltelevision set 1, the input terminals 13, 15 of the common mode filter10 are connected to corresponding terminals of the joining terminal part6.

The structure and operation of the common mode filter 10 will bedescribed below with reference to FIG. 3. FIG. 3 is a schematic diagramto illustrate the operation of the common mode filter.

The common mode filter 10 is constructed in a configuration wherein twoconductors 17, 18 electrically isolated from each other are wound byseveral turns on a ferrite core 19. The conductor 17 constitutes thefirst inductor 11, and the conductor 18 the second inductor 12. Theshape of the ferrite core 19 does not always have to be limited to thering shape as illustrated.

In the present embodiment, the common mode filter 10 is used in thedifferential mode for signals. In the differential mode, as shown in (a)in FIG. 3, signals SI are fed as mutually opposite signals into theconductors 17, 18. For this reason, magnetic fluxes F1, F2 induced inthe ferrite core 19 by the respective conductors 17, 18 are mutuallyopposite ones and act so as to cancel each other. Therefore, there islittle impedance (inductance) due to a magnetic field MF made by theconductors 17, 18, and the signals SI are outputted with littleattenuation.

On the other hand, the common mode filter 10 is used in the common mode,for common mode noise CN. In the common mode, as shown in (b) in FIG. 3,common mode noise CN appears in the same direction of the conductors 17,18. For this reason, magnetic fluxes F1, F2 induced in the ferrite core19 by the respective conductors 17, 18 are those in the same directionand act so as to reinforce each other. Therefore, the impedance(inductance) due to the magnetic field MF made by the conductors 17, 18is high, so that little common mode noise CN is outputted. In thismanner, the common mode filter 10 is able to attenuate the noise.

Reference is made again to FIG. 2. The third inductor 21 hasinput/output terminals 23, 24. The input terminal 23 of the thirdinductor 21 is connected to the output terminal 14 of the common modefilter 10. The third inductor 21 is electrically connected in series tothe first inductor 11. The third inductor 21 is substantially notmagnetically coupled to the first inductor 11. The first varistor 31 hasinput/output terminals 33, 34. The input terminal 33 of the firstvaristor 31 is connected to the output terminal 24 of the third inductor21. The output terminal 34 of the first varistor 31 is connected to theground potential. In this configuration, the first varistor 31 islocated posterior to the first inductor 11 and the third inductor 21 andis electrically connected in parallel to the first inductor 11 and thirdinductor 21. The third inductor 21 is located between the first inductor11 and the first varistor 31.

The fourth inductor 22 has input/output terminals 25, 26. The inputterminal 25 of the fourth inductor 22 is connected to the outputterminal 16 of the common mode filter 10. The fourth inductor 22 iselectrically connected in series to the second inductor 12. The fourthinductor 22 is substantially not magnetically coupled to the secondinductor 12. The second varistor 32 has input/output terminals 35, 36.The input terminal 35 of the second varistor 32 is connected to theoutput terminal 26 of the fourth inductor 22. The output terminal 36 ofthe second varistor 32 is connected to the ground potential. In thisconfiguration, the second varistor 32 is located posterior to the secondinductor 12 and the fourth inductor 22 and is electrically connected inparallel to the second inductor 12 and the fourth inductor 22. Thefourth inductor 22 is located between the second inductor 12 and thesecond varistor 32.

The common mode filter 10 can be, for example, one of the common modefilters in the ACM series available from TDK Corp. The first and secondvaristors 31, 32 can be, for example, multilayer chip varistors in theAVR series available from TDK Corp.

In the first embodiment, as described above, the common mode filter 10(first and second inductors 11, 12) is inserted anterior to the firstand second varistors 31, 32, and the third and fourth inductors 21, 22are inserted each between the common mode filter 10 and the first andsecond varistors 31, 32. This enables the circuit to suppress thereduction of the characteristic impedance due to the first and secondvaristors 31, 32.

In the first embodiment the common mode filter 10 is inserted anteriorto the first and second varistors 31, 32. This permits the signal fromthe DVD player 2 to be fed through the HDMI cable 3 and signaltransmission circuit SC1 to the digital television set 1, with littleexternal noise.

Next, configurations in modification examples of the signal transmissioncircuit SC1 according to the first embodiment will be described on thebasis of FIGS. 4 to 6. FIGS. 4 to 6 are illustrations showing themodification examples of the signal transmission circuit according tothe first embodiment.

In the modification example shown in FIG. 4, the HDMI cable 3 isprovided with the signal transmission circuit SC1.

In the modification example shown in FIG. 5, the DVD player 2 isprovided with the signal transmission circuit SC1 in the output partthereof.

In the modification example shown in FIG. 6, the joining terminal part 6(connector) of the HDMI cable 3 is provided with the signal transmissioncircuit SC1. Instead of the configuration wherein the joining terminalpart 6 of the HDMI cable 3 is provided with the signal transmissioncircuit SC1, the joining terminal part 5 (connector) of the HDMI cable 3may be provided with the signal transmission circuit SC1.

In all the modification examples shown in FIGS. 4 to 6, the circuit isable to suppress the reduction of the characteristic impedance due tothe first and second varistors 31, 32.

Second Embodiment

Next, a configuration of a signal transmission circuit according to thesecond embodiment will be described on the basis of FIG. 7. FIG. 7 is acircuit diagram showing the signal transmission circuit according to thesecond embodiment.

The digital television set 1 is equipped with the signal transmissioncircuit SC2 in the input part thereof, as in the first embodiment. Thesignal transmission circuit SC2, as shown in FIG. 7, is comprised of acommon mode filter 10, third and fourth inductors 21, 22, first andsecond varistors 31, 32, and fifth and sixth inductors 41, 42.

The fifth inductor 41 has input/output terminals 43, 44. The inputterminal 43 of the fifth inductor 41 is connected to the output terminal24 of the third inductor 21. The fifth inductor 41 is electricallyconnected in series to the first inductor 11 and third inductor 21. Inthis configuration, the fifth inductor 41 is located posterior to thefirst varistor 31.

The sixth inductor 42 has input/output terminals 45, 46. The inputterminal 45 of the sixth inductor 42 is connected to the output terminal26 of the fourth inductor 22. The sixth inductor 42 is electricallyconnected in series to the second inductor 12 and fourth inductor 22. Inthis configuration, the sixth inductor 42 is located posterior to thesecond varistor 32.

In the second embodiment, as described above, the fifth and sixthinductors 41, 42 are inserted posterior to the first and secondvaristors 31, 32, respectively. This permits the circuit to furthersuppress the reduction of the characteristic impedance due to the firstand second varistors 31, 32.

The signal transmission circuit SC2 may be provided in the HDMI cable 3,in the DVD player 2, or in the joining terminal part 5 or 6 (connector),as shown in FIGS. 4 to 6. In this case, it is also feasible to furthersuppress the reduction of the characteristic impedance due to the firstand second varistors 31, 32.

The following will describe by specific examples that the first andsecond embodiments are able to suppress the reduction of thecharacteristic impedance due to the first and second varistors. Thecharacteristic impedance of the signal transmission circuit herein ismeasured by the TDR (Time Domain Reflectometry) method. The TDR methodis a measurement method of feeding a step pulse onto a transmissionline, and measuring a pulse reflected at a discontinuity of thecharacteristic impedance, thereby measuring the characteristic impedanceof the transmission line.

First, the measurement environment by the TDR method will be describedon the basis of FIG. 8. In each measurement environment shown in FIG. 8,a high-speed oscilloscope 50 and a receiver IC 52 are connected througha transmission channel 54. The transmission channel 54 has a coaxialcable 56 and a signal transmission circuit 58. The high-speedoscilloscope 50 has a TDR module 51. The high-speed oscilloscope 50 isconnected through the TDR module 51 with the coaxial cable 56, and theother end of the coaxial cable 56 is connected to the signaltransmission circuit 58. The receiver IC 52 is connected to the otherend of the signal transmission circuit 58.

The high-speed oscilloscope 50 used herein is Agilent 86100:wide-bandwidth oscilloscope available from Agilent Technologies, Inc.The TDR module 51 used herein is the 54754 differential TDR plug-inmodule available from Agilent Technologies, Inc. The receiver IC 52 hasthe infinitely large input impedance with power off, and reflects 100%of the signal from the high-speed oscilloscope 50. The coaxial cable 56consists of two differential signal lines and each signal line has thecharacteristic impedance of 50 Ω. For this reason, the totalcharacteristic impedance of the coaxial cable 56 is 100 Ω.

Next, the measurement method by the TDR method will be described on thebasis of FIGS. 8 and 9. First, the high-speed oscilloscope 50 generatesan input voltage step Ei and outputs this input voltage step Ei to thetransmission channel 54. Where there is no discontinuity ofcharacteristic impedance on the transmission channel 54, the inputvoltage step Ei is reflected by the receiver IC 52 as it is. In thiscase, the high-speed oscilloscope 50 displays only the input voltagestep Ei, as shown in (a) in FIG. 9. On the other hand, where there is adiscontinuity of characteristic impedance on the transmission channel54, part of the input voltage step is reflected at the discontinuity. Inthis case, the high-speed oscilloscope 50 displays a reflected wave Eralgebraically added to the input voltage step Ei, as shown in (b) inFIG. 9. We can determine the position of the discontinuity ofcharacteristic impedance and the value of the characteristic impedancefrom this result. Specifically, the position of the discontinuity ofcharacteristic impedance can be determined based on a time T up to themeasurement of the reflected wave Er and the characteristic impedance atthe discontinuity can be determined based on the value of the reflectedwave Er.

The common mode filter used herein was ACM2012D-900 (available from TDKCorp.). The characteristic impedance of ACM2012D-900 is 100 Ω. Thecutoff frequency of ACM2012D-900 is 3.5 GHz. The first and secondvaristors used herein were AVR161AIR1 (available from TDK Corp.). Thecapacitance of AVR161AIR1 is 1.1 pF. The third to sixth inductors usedherein were the MLK1005 series (available from TDK Corp.).

The measurement results are shown in FIGS. 10 to 12.

Reference is made to FIG. 10. Characteristic I1 represents themeasurement result in a case where the signal transmission circuit 58has the first and second varistors but has none of the common modefilter and the third to sixth inductors. It is seen from thecharacteristic I1 that the characteristic impedance is lowered byinfluence of the first and second varistors, so as to cause an impedancemismatch.

Characteristic I2 represents the measurement result in a case where thesignal transmission circuit 58 has the first and second varistors andthe common mode filter but has none of the third to sixth inductors. Thesignal transmission circuit 58 was so constructed that the separation onthe transmission lines between the output terminals of the common modefilter and the input terminals of the first and second varistors, i.e.,the temporal length between the output terminals of the common modefilter and the input terminals of the first and second varistors was setto 23 ps.

It is seen from the characteristic I2 that the characteristic impedanceof the signal transmission circuit 58 is within the range of 100 Ω±15%but the characteristic impedance remains lowered due to the influence ofthe first and second varistors.

Characteristics I3-I5 represent the measurement results in cases wherethe signal transmission circuit 58 has the first and second varistors,the common mode filter, and the third and fourth inductors, i.e., wherethe signal transmission circuit 58 has the same configuration as thesignal transmission circuit SC1 of the first embodiment described above.The characteristic I3 represents the measurement result in the casewhere the inductances of the third and fourth inductors are 1.0 nH. Thecharacteristic I4 represents the measurement result in the case wherethe inductances of the third and fourth inductors are 1.5 nH. Thecharacteristic I5 represents the measurement result in the case wherethe inductances of the third and fourth inductors are 2.2 nH. The signaltransmission circuit 58 was so constructed that the separation on thetransmissions lines between the output terminals of the common modefilter and the input terminals of the third and fourth inductors, i.e.,the temporal length between the output terminals of the common modefilter and the input terminals of the third and fourth inductors was setto 20 ps. Likewise, the separation on the transmission lines between theoutput terminals of the third and fourth inductors and the inputterminals of the first and second varistors, i.e., the temporal lengthbetween the output terminals of the third and fourth inductors and theinput terminals of the first and second varistors was set to 0 ps.

It is seen from the characteristics I3-I5 that the reduction of thecharacteristic impedance due to the first and second varistors issuppressed well.

It is seen from the characteristic I5 that, where the inductances of thethird and fourth inductors are 2.2 nH, the characteristic impedance islowered at the position of the first and second varistors, but thecharacteristic impedance becomes increased in another region. Theappearance of the region with high characteristic impedance isconceivably attributable to the inductances of the third and fourthinductors. Therefore, the inductances of the third and fourth inductorsare preferably in the range of 1 to 2 nH.

Next, reference is made to FIG. 11. Characteristic I6 and characteristicI7 represent the measurement results in cases where the signaltransmission circuit 58 has the first and second varistors, the commonmode filter, and the third to sixth inductors, i.e., where the signaltransmission circuit 58 has the same configuration as the signaltransmission circuit SC2 of the second embodiment described above. Thecharacteristic I6 represents the measurement result in the case wherethe inductances of the third to sixth inductors are 1.0 nH. Thecharacteristic I7 represents the measurement result in the case wherethe inductances of the third and fourth inductors are 1.0 nH and wherethe fifth and sixth inductors are bypassed. The signal transmissioncircuit 58 was so constructed that the separation on the transmissionlines between the output terminals of the common mode filter and theinput terminals of the third and fourth inductors, i.e., the temporallength between the output terminals of the common mode filter and theinput terminals of the third and fourth inductors was set to 0 ps.Likewise, the separation on the transmission lines between the outputterminals of the third and fourth inductors and the input terminals ofthe first and second varistors, i.e., the temporal length between theoutput terminals of the third and fourth inductors and the inputterminals of the first and second varistors was set to 0 ps. In the samemanner, the separation on the transmission lines between the inputterminals of the first and second varistors and the input terminals ofthe fifth and sixth inductors, i.e., the temporal length between theinput terminals of the first and second varistors and the inputterminals of the fifth and sixth inductors was set to 0 ps.

It is seen from the characteristic I6 that the reduction of thecharacteristic impedance due to the first and second varistors is moresuppressed.

Next, reference is made to FIG. 12. Characteristics I8-I10 represent themeasurement results in cases where the signal transmission circuit 58has the first and second varistors, the common mode filter, and thethird to sixth inductors, i.e., where the signal transmission circuit 58has the same configuration as the signal transmission circuit SC2 of thesecond embodiment described above. The characteristic I8 represents themeasurement result in the case where the inductances of the third andfourth inductors are 1.5 nH and where the inductances of the fifth andsixth inductors are 1.0 nH. The characteristic I9 represents themeasurement result in the case where the inductances of the third tosixth inductors are 1.5 nH. The characteristic I10 represents themeasurement result in the case where the inductances of the third andfourth inductors are 1.5 nH and where the fifth and sixth inductors arebypassed. The signal transmission circuit 58 was so constructed that theseparation on the transmission lines between the output terminals of thecommon mode filter and the input terminals of the third and fourthinductors, i.e., the temporal length between the output terminals of thecommon mode filter and the input terminals of the third and fourthinductors was set to 0 ps. Likewise, the separation on the transmissionlines between the output terminals of the third and fourth inductors andthe input terminals of the first and second varistors, i.e., thetemporal length between the output terminals of the third and fourthinductors and the input terminals of the first and second varistors wasset to 0 ps. In the same manner, the separation on the transmissionlines between the input terminals of the first and second varistors andthe input terminals of the fifth and sixth inductors, i.e., the temporallength between the input terminals of the first and second varistors andthe input terminals of the fifth and sixth inductors was set to 0 ps.

It is seen from the characteristics I8 and I9 that the reduction of thecharacteristic impedance due to the first and second varistors is moresuppressed.

The above confirmed the usefulness of the first and second embodiments.

As apparent from the above measurement results, the inductances of thethird to sixth inductors are preferably smaller than 10 nH and morepreferably in the range of 1-2 nH. This is because the inductances ofthe third to sixth inductors can produce a region with highcharacteristic impedance, so as to make the impedance matchinginsufficient.

The separation on the transmission lines between the output terminals ofthe common mode filter and the input terminals of the third and fourthinductors, the separation on the transmission lines between the outputterminals of the third and fourth inductors and the input terminals ofthe first and second varistors, and the separation on the transmissionlines between the input terminals of the first and second varistors andthe input terminals of the fifth and sixth inductors are preferably asshort as possible. The reason for it is that the transmission linesbetween terminals (e.g., conductor patterns on a substrate) have theirrespective inductances and capacitances and that these inductances andcapacitances can be factors to impede the impedance matching.

Where the common mode filter is used as a noise filter, a capacitor isconnected between the signal lines in certain cases (e.g., reference ismade to Japanese Patent Application Laid-Open No. 2004-40444). However,if a capacitor is connected between the signal lines in the first andsecond embodiments, an unwanted capacitance will be made, so as to failto achieve impedance matching. Therefore, the first and secondembodiments are provided without the capacitor between the signal lines.

The preferred embodiments of the present invention were described above,but it is noted that the present invention is not always limited tothese embodiments. For example, the signal transmission circuit SC1, SC2does not always have to be located at the positions described above, butmay be located anywhere after the output from the DVD player 2 andbefore the first circuit of the digital television set 1. The DVD player2 may be another source device such as a personal computer or a set-topbox. The HDMI cable 3 may be a cable compliant with the standards suchas DVI, USB, or IEEE. The digital television set 1 may be another sinkdevice such as an LCD monitor or a projector.

The first and second embodiments employed the varistors as the first andsecond capacitive elements, but the first and second capacitive elementsmay be other capacitive elements such as Zener diodes.

The common mode filter 10 may be any other filter than the wound commonmode filter in which two conductors electrically isolated from eachother are wound by several turns on the ferrite core; for example, itmay be a multilayer common mode filter, a multilayer common mode filterwith conductor patterns formed by thin-film forming technology, or thelike.

In a case where the input/output terminals of the common mode filter 10are constructed of terminal electrodes of metal, the terminal electrodesmay be used as the third and fourth inductors 21, 22. In this case, itis necessary to set the size of the terminal electrodes (e.g., theelectrode width or the like) so as to achieve the aforementionedimpedances.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1. A signal transmission circuit comprising: first and second inductorsmagnetically coupled to each other; a first capacitive element locatedposterior to the first inductor and electrically connected in parallelto the first inductor; a second capacitive element located posterior tothe second inductor and electrically connected in parallel to the secondinductor; a third inductor located between the first inductor and thefirst capacitive element and electrically connected in series to thefirst inductor; and a fourth inductor located between the secondinductor and the second capacitive element and electrically connected inseries to the second inductor.
 2. The signal transmission circuitaccording to claim 1, further comprising: a fifth inductor locatedposterior to the first capacitive element and electrically connected inseries to the third inductor; and a sixth inductor located posterior tothe second capacitive element and electrically connected in series tothe fourth inductor.
 3. An electronic device comprising the signaltransmission circuit as defined in claim
 1. 4. An electronic devicecomprising the signal transmission circuit as defined in claim
 2. 5. Acable comprising the signal transmission circuit as defined in claim 1.6. A cable comprising the signal transmission circuit as defined inclaim
 2. 7. A connector comprising the signal transmission circuit asdefined in claim
 1. 8. A connector comprising the signal transmissioncircuit as defined in claim 2.